Thermal printers are often used in conventional office and laboratory settings. In these settings, the operators of the thermal printers are usually not printing machine specialists but are instead people who have other job functions. In order for a thermal printer to be successfully used in these settings there must be a simple and easily understood method for loading printing material into the printers.
The materials to be loaded in thermal printers are receivers (e.g., either paper sheets or transparency sheets) and dye-donor film. A typical dye-donor color film consists of repeating patches of colored dye formed on a flexible film.
In operation, a thermal printer uses a programmable print head to progressively form an image on a moving receiver in a series of successive lines of print. The dye-donor film is sandwiched between the print head and the moving receiver. The print head heats selected portions of successive lines of an advancing dye patch as the dye-donor film moves along between the print head and the receiver. A computer generated image is thus progressively transferred to the receiver as the receiver and dye patch are moved together under the print head.
The dye-donor film used in a typical thermal printer can be packaged in cartridges that can be easily loaded into the printers. The cartridges are arranged so that an unused portion of the film is successively unrolled from a spool in the cartridge and a used portion of the film is re-rolled onto another spool. Each cartridge typically contains enough dye-donor film to create one-hundred images.
In many applications, a printer is used to make a plurality of images without interruption. In this context, an operator must be assured that the printer is loaded with a cartridge that has a sufficient amount of remaining unused dye-donor film to permit the completion of the entire succession of images. However, the dye-donor film is very thin and it is therefore extremely difficult to estimate the amount of remaining film in the unused section of a cartridge.
In some prior art thermal printers, a reset counter has been used to count the number of patches used in a cartridge. This system is workable only if a cartridge is put into a printer and allowed to remain until completely used. In practice, however, thermal printers are not used in a manner that simply consumes dye-donor film cartridges end to end. A more typical use pattern for a thermal printer involves frequent removal and replacement of partially used cartridges of dye-donor film. For example, the thermal printer may be used to make a number of prints from a black ribbon cartridge. The black ribbon cartridge might then be replaced with a full-color cartridge for a few prints and at a later time or on a later day the black ribbon cartridge might again be placed into the thermal printer. At other times, when confidentiality is a factor, a cartridge might be removed after making only one confidential print. The cartridges retain an imprint of a transferred image and therefore the cartridges must be kept secure when confidential images are being produced.
When cartridges are interchanged on thermal printers, a resetting counter on a printer is virtually useless in determining the number of dye patches remaining in a cartridge. Thus there is a need for a system which indicates how many dye patches remain on a previously used cartridge.
An ostensibly simple method of filling this need is to put identifying numbers on each set of dye patches so that a user of the thermal printer can read the number of remaining patches. However, this solution to the problem has not been applied to dye-donor films in thermal printers because of the unique physical properties of the dye-donor films. In order for a thermal-printing operation to be capable of producing high resolution images, the dye-donor film must have two critical characteristics. First, a supporting web of the film must be extremely thin so that heat transfer can rapidly take place through the film. Secondly, the dye coating on the film must be extremely uniform so that a predictable amount of the dye is transferred in response to a particular amount of energy applied to the print head.
With these critical characteristics to be met, the prior-art techniques for making dye-donor film consisted of coating a clear web with dye in a gravuring process, slitting the coated web into narrower strips of dye-donor film and packaging the film into cartridges. In this prior-art coating technique, it has not been possible to perform sequential numbering of dye patches on the dye-donor film.
The major reason for this previous difficulty resides in the method by which the dye-donor film is coated with dye patches to achieve these important characteristics. A web of polyethylene terephthalate film which typically is only 0.00025 inches thick is used as a base for the dye-donor film. The web is coated with dye in a precisely controlled gravure-deposition process. Within the gravure process the web is advanced with a carefully controlled speed so that a uniform thickness of dye is deposited on the film by engraved dye deposition rollers. The engraved portions of the deposition rollers are carefully sized so that coordinated rotation of a series of these rollers makes a perfectly spaced pattern of uniformly thick dye patches on the thin base film. Any efforts to put ink or dye coding on the web during gravuring have been unsuccessful because of the risk of contaminating the gravuring operation. Efforts to create coding by punching holes in the web have also been unsuccessful because the web is extremely thin and coding holes create a high risk of web breakage or tearing.
An additional problem in producing dye-donor film relates to uniformity of the dye patches from one lot of dye-donor film to the next. In spite of extraordinary effort to control dye thickness and dye uniformity, some variations in the optical color-balance of the dye-donor film still occurs. The thermal printers which use these dye-donor films are calibrated to perform well when a dye-donor film of nominal color-balance is put into the printer. Thus the printers perform at less than their peak capability when using dye-donor film having a color-balance that deviates from a nominal value.